Using Lewis strain rats, experiments will be carried out to identify IgE-specific regulator T cells and to detect soluble T cell factors which regulate the IgE response. The nature of IgE-potentiating factor from lymphocytes of rats infected with Nippostrongylus brasiliensis and mechanisms through which the factor will recognize IgE-precursor B cells will be investigated. The relationship of this factor with Fc receptors on T cells will be studied. We shall also look for IgE-specific suppressor factor epsilon from rat lymphocytes.